1. Field of the Invention
The present invention relates to a method of synthesizing a glycol ester represented by formula (3) from a glycol monoester represented by formula (1) and a linear aliphatic carboxylic acid represented by formula (2) in the presence of a Lewis acid type catalyst utilizing reactive distillation technique in which a reaction time can be significantly reduced by rapidly removing water produced during the reaction:HO—R1—O—C(═O)—R2  (1)R3—C(═O)—OH  (2)R3—C(═O)—O—R1—O—C(═O)—R2  (3)where R1 is a C1-C16 alkylidene group, R2 is a C1-C16 alkyl group, and R3 is a C3-C16 linear alkyl group.
2. Description of the Related Art
Generally, in the case of the same molecular weight, glycol diesters produced by the reaction between glycol monoesters and linear organic acids have superior physical properties than glycol diesters produced by the reaction between glycol monoesters and branched organic acids due to physical properties of the linear organic acids. For example, viscosity, migration-resistance in the application processing test of polymers such as polyvinyl chloride (PVC), and etc.
Glycol diesters are produced through an ester reaction from alcohols and acids in the presence of catalysts, in which water is also produced as a by-product together with the desirable products. During the reaction, water increases over time. In the reversible ester reaction, due to the presence of water, the reaction rate is reduced and the activity of the used Lewis acid catalyst is degraded. Thus, in order to reduce the reaction time by increasing the reaction rate, it is important to rapidly separate and remove water produced during the reaction from reactants. Studies on a method of removing water produced during the reaction, at fastest rate, have been conducted. Also, a method of increasing the yield of glycol esters by optimizing a catalyst and operating conditions, etc. has also been studied. Conventional technologies of synthesizing glycol diesters are as follows.
Japanese Patent Laid-Open Publication No. 49-94621 discloses the reaction of 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate and isobutyric acid using tetraisopropyl titanate as a catalyst. In the method, xylene is used as a solvent and water produced during the reaction is removed using azeotropic distillation. In the publication, it is described that the reaction time is 18 hours which is significantly shortened time compared to 24 hours required in other conventional methods, and the yield of 2,2,4-trimethyl-1,3-pentanediol diisobutyrate is 95%. However, this technology further requires a process of recovering the solvent after the reaction and does not achieve significant reduction in the reaction time.
In DE Pat. No. 3,102,826, isobutyraldehyde is used as a starting material and para-toluenesulfonic acid which is a strong acid is used as a reaction catalyst. As a result, the reaction is completed in only about 4 hours. Although the reaction time is somewhat shortened, the selectivity to 2,2,4-trimethyl-1,3-pentanediol diisobutyrate is 78% and reaction process yield was low (61%).
U.S. Pat. No. 5,180,847 teaches that isobutyraldehyde is used as a starting material and an alkali metal hydroxide which is a strong base is used as a catalyst to produce about 23% of 2,2,4-trimethyl-1,3-pentanediol, about 28% of 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate and 41% of 2,2,4-trimethyl-1,3-pentandiol diisobutyrate. In this case, three useful components can be simultaneously obtained, but when 2,2,4-trimethyl-1,3-pentanediol diisobutyrate is desired as a final product, the yield of the final product is low.
As described above, when a Lewis acid type catalyst is used to increase the yield of glycol diester such as 2,2,4-trimethyl-1,3-pentandiol diisobutyrate, the reaction time is long. Meanwhile, when a strong acid or strong base catalyst is used to shorten the reaction time, the selectivity to glycol diester and the yield thereof are low. Thus, there is a demand for a process capable of maximizing the yield of glycol diester and shortening the reaction time.
The inventors of the present invention discovered that the selectivity to glycol diester can be maximized by using a Lewis acid type catalyst which is a weak acid, for example, tetraisopropyl titanate, tetra-n-butyl titanate, tetra 2-ethylhexyl titanate, etc., and reactive distillation through a distillation column installed in a reactor is utilized to effectively remove water produced during the reaction under the condition that an excess of a linear aliphatic carboxylic acid is used, thereby achieving significant reduction in the reaction time. Thus, the present invention is completed.